A DC-DC converter circuit is a type of switching circuit that regulates an output voltage from a given input voltage. The circuit may step-up (e.g. Boost converter) or step-down (e.g. Buck converter) the voltage. Switching circuits come in two varieties, standard and synchronous. FIG. 1A illustrates a prior art standard Buck DC-DC converter circuit 100. Standard Buck converters use a control switching transistor 102 to regulate voltage and current flow in the upper side, while a diode 104 is used for the lower side of the circuit. A periodic signal, such as a Pulse Width Modulated (PWM) signal exhibiting a duty cycle (D), allows control and regulation of the output voltage by exercising the control switch in accordance with the duty cycle of the PWM signal. The duty cycle represents the fraction of a switching period during which the control switching transistor 102 is turned on. The output voltage (VOUT) 120 is directly proportional to the product of the input voltage (VIN) 108 and the duty cycle (D). A driver 116 alternately switches the upper-side transistor 102 on and off in step with the PWM signal, thereby regulating the time-averaged voltage at the switching node 106. A forward biased diode 104, facilitates the continuous flow of inductor current when the control transistor 102 is switched off. The diode 104 is customarily referred to as a “freewheeling diode”, since it circulates positive output inductor 110 current flow (flowing toward VOUT 120) while the control switching transistor 102 is in an off state. The combination of the transistor 102 and diode 104, in response to the PWM signal, operate to regulate the voltage level VOUT 120 across the load, RL 114. A voltage source provides input voltage at VIN 108. The switching node 106 between the control switch 102 and the freewheeling diode 104 is connected to the converter's output through a low-pass filter comprising an inductor 110 and an output capacitor 112. VOUT 120 is referenced to ground 118.
FIG. 1B illustrates a prior art synchronous DC-DC converter circuit 122. This type of converter circuit 122 uses a half-bridge switch configuration, and therefore has two switching transistors, which are known as the upper-side (or control) transistor 102 and the lower-side (or synchronous) transistor 124. The two transistors are actively controlled to alternately switch on and off out of phase with each other, regulating the output voltage VOUT 120. A voltage source provides input voltage at VIN 108. The switching node 106 between the two devices is connected to the converter's output through a low-pass filter comprising an output inductor 110 and an output capacitor 112. VOUT 120 is referenced to ground 118. A driver 116 provides on/off control for the switching transistors 102 and 124, in response to a periodic control signal such as a PWM signal.
The transistor switches can be fabricated as either enhancement-mode (E-mode) devices, which are off-state at zero applied gate bias, or depletion-mode (D-mode) devices, which are on-state at zero applied gate bias.
Silicon (Si) based voltage converters typically use enhancement mode MOSFETs for the transistor switches 102, and 124, and for drivers. A Gallium-Arsenide (GaAs) depletion mode power FET transistor is also a 3-terminal device. There is a channel running between drain (D) and source (S), whose conduction is controlled by the gate (G) potential. The gate-to-source (or gate-to-drain) junction V-I characteristics resemble very much that of a diode junction, ranging from a Schottky to a high-threshold P-N diode. A N-channel depletion mode FET has a negative threshold for D-S conduction and is on when no voltage is applied to the gate, turning off when a bias more negative than its threshold is applied G-S. As the G-S bias increases above the threshold voltage, the D-S channel resistance decreases in proportion to the increase in bias differential. G-S bias is ultimately limited in the positive domain by the forward biasing of the G-S junction.
Compound semiconductors, such as GaAs III-V materials, offer high frequency transistor switching capabilities, however, their G-S control junctions (some resembling Schottky junctions) can be leakier than doped P-N silicon counterparts. Engineering the control circuits to manage the junction leakage issue in the absence of existing negative bias supplies, is substantially more difficult. Also, synchronous converter circuits built using all D-mode switching transistors may conduct current in an uncontrolled and potentially wasteful and/or damaging fashion upon system start-up, before the control electronics can apply nominal voltages to the transistor gate electrodes.
The need for additional circuit components, design complexity and the normally inexistent negative bias supplies in typical applications has deterred prior commercialization of D-mode gate drivers.